A disk drive is a data storage device that stores digital data in substantially concentric tracks on a data storage disk therein. During disk drive operation, the data storage disk is rotated about an axis while a transducer is used to read and/or write data from/to a target track of the disk.
A servo control loop is used to position the transducer above the target track while the data transfer is taking place. The servo control loop uses servo data read from a surface of the data storage disk as position feedback to maintain the transducer in a substantially centered position above the target track that is dictated by the mechanical properties of the drive.
When a transducer moves off-track during a write operation, there is a chance that the transducer might inadvertently write data on or near an adjacent track, thus corrupting the data written on the adjacent track (encroachment). In addition, the data that is written off-track by the transducer may be difficult or impossible to read during a subsequent read operation on the present track due to its off-track position. Thus, an off-track threshold value is typically defined in a disk drive that indicates an off-track transducer position beyond which write operations will be suspended. If the transducer goes beyond this off-track position window during a write operation, the write operation is suspended until the transducer again comes within the specified positional window about the target track.
The off-track threshold has traditionally been determined during disk drive development based upon collected (worst case) off-track capability (OTC) data and estimates of transducer positioning error. A single off-track threshold value was then used for all transducers within all drives in a production run. During disk drive test, if the OTC of the transducers in a particular drive were all within a specified range and the measured position error of the drive was also within a corresponding range, the disk drive would be passed (accepted). It would thus be assumed that the off-track threshold programmed into the drive would be sufficient to prevent adjacent track data corruption and unreadable off-track data. If the OTC of a transducer was not within the specified range, the transducer would not be used in a disk drive. Similarly, if a particular drive displayed greater than a predetermined position error, the drive would not be used (failed). As such, the greater the number of disk drive units that are left unused during the manufacturing process, the greater the overall manufacturing costs.
Another disadvantage of conventional techniques for measuring off-track threshold values is that the risks of hard error (unrecoverable data error) rates are assessed by squeezing both tracks adjacent to a test data track in the presence of Repeatable Run Out (RRO) RRO. The data track is written first and the adjacent tracks to the sides of the data track are then written in the presence of a squeeze amount, and the read error rate (Bit Error Rate) is measured on the data track. The amount of squeeze present on any individual test data track or disk drive during actual writing of the adjacent track is enhanced by the actual Repeatable Run Out (RRO) and Non Repeatable Run Out (NRRO) of the test tracks. The amount of squeeze set in such disk drives is determined by a selected off-track threshold (write fault threshold value or Write Fault Limit (WFL)) and an assumption of worst case NRRO and RRO (i.e., worst case position error signal as root mean square sum of the assumed worst case RRO and NRRO). This is used as a screen to insure that no disk drives are passed that will generate hard errors due to track encroachment.
In the conventional measurement process it is assumed that every disk drive will only encounter the assumed worst case position error signal on the test data track. Therefore, each disk drive is only guaranteed to perform without hard read errors (unrecoverable read errors) when the squeeze amount is combined with the worst case position error signal (PES) effects. The effects of RRO are distributed in a manner dependent on spindle quality and other mechanical issues which are not represented as worst case in all drives. Further, RRO is further distributed across the tracks of a disk in a non-uniform manner. Hence, diagnostics that test disk drives, with less than a worst case track position (e.g., 99.73% of a 3-sigma distribution of tracks) can not accurately predict the disk drive tendency to create hard errors with a fixed amount of squeeze. Conversely if a track is selected that is an extreme outlier in the RRO distribution, the drive is failed for insufficient margin. As such, there are disk drives that pass those diagnostic tests, but fail in the field during customer use due to hard errors that are not recovered, and there are disk drives that fail the diagnostic tests but will not produce errors in the field.
There is, therefore, a need for a method and apparatus for accurately predicting the risk of hard errors in a disk drive due to encroachment effects resulting from vibration, shock, seek settle, and recording head write and read widths). There is need for such method and apparatus to increase yields during the disk drive manufacturing process without compromising disk drive performance.